If you buy tapered leaders with loops already in them, check them before using. Whomever ties the loops for Rio and Cortland salt water tapered leaders does not know how to cinch up a knot. Every single one I have tested has come apart at below the leader strength. Do not be afraid of really cinching up the butt sections especially - but the same goes for your terminal knots. It they do not cinch up, you are using too many wraps or lousy lubricant.

As for loop to loop vs direct connection, I can tie a new tippet onto a leader faster than I can change a loop to loop out and re-coil or deal with the old one. Besides, no one makes a leader long enough for my purposes anyhow.

For salt water I have gone to a nearly "knotless" connection using braided mono from line to leader since I am stowing and unstowing the rod often, and using very long leaders which need to be reeled partway in while fishing alone and landing fish.

Nail knots go through the guides very easily - especially if you fair them in with knot sense or some other UV cure flexible "adhesive" (pliobond is the old standby but very messy).

Don't worry about the twisted braid as others have said. After it's fished and stored, it will likely have a bunch of little squiggles in it and should be re-spooled next spring because it will be loose. Fix it then.